A Justice Department official said that Holder told the governors in a joint phone call early Thursday afternoon that the department would take a “trust but verify approach” to the state laws. DOJ is reserving its right to file a preemption lawsuit at a later date, since the states’ regulation of marijuana is illegal under the Controlled Substances Act.

Though the Department of Justice has reserved the right to block the laws entirely, this reprieve gives legalization advocates the chance to prove that the drug can be taxed and regulated in much the same way as alcohol.

Marijuana legalization is supported by most Americans, including a vast majority of voters 65 and younger.

The legalization of medical and now recreational marijuana has become the rare states’ rights issue that tends to unite progressives and libertarians. And now that the Obama administration has taken a hesitant stand in favor of state-by-state legalization, it will be fascinating to watch a Republican candidate, like maybe Senator Rand Paul (R-KY), take on his party’s anti-drug orthodoxy — possibly defended by, say, Rick Santorum — in a presidential primary.